1. Field of the Invention
The present invention generally relates to piezoelectric speakers, and more specifically, to a piezoelectric speaker using a piezoelectric element and having an improved sound pressure characteristic, preferably for use as a tweeter.
2. Description of the Related Art
FIG. 4 is a sectional view of a piezoelectric speaker of the related art.
Referring to FIG. 4, a piezoelectric speaker 40 has a piezoelectric vibrator 1. The piezoelectric vibrator 1 includes a piezoelectric element 2 which has a dome shape with a hollow portion and which has a uniform thickness, a first electrode 3 disposed on the inner surface of the piezoelectric element 2, and a second electrode 4 disposed on the outer surface of the piezoelectric element 2. The dome-shaped piezoelectric element 2 is polarized in the thickness direction.
In the piezoelectric speaker 40 constructed in this manner, when an external signal source 5 applies a signal between the first electrode 3 and the second electrode 4, the entire surface of the piezoelectric element 2 expands and contracts correspondingly. The expansion and contraction thereof causes a “breathing vibration” of the entire dome-shaped piezoelectric element 2, thereby generating sound.
In the piezoelectric speaker 40 of the related art, however, sounds 6a and 6b which are generated from the inner surface of the dome-shaped piezoelectric vibrator 1 are transmitted inwardly, that is, toward the opposite side, and thus do not contribute to the level of sound emitted from the piezoelectric speaker 40. Consequently, only sound pressure provided by a sound 8 generated from the outer surface is available, and hence the piezoelectric speaker 40 requires an increase in the level of sound pressure.
FIG. 3 shows the sound pressure vs. frequency characteristic of the piezoelectric speaker 40 of the related art, in which a dome-shaped piezoelectric vibrator having a diameter of 20 mm is used. The vertical axis indicates the sound pressure level (S.P.L.) and the horizontal axis indicates the frequency. FIG. 3 also shows the sound pressure characteristic of a piezoelectric speaker 10 of preferred embodiments of the present invention, and the details thereof will be described later. The sound pressure characteristic of the piezoelectric filter 40 of the related art is indicated by a dotted line and the sound pressure characteristic of the piezoelectric speaker 10 of preferred embodiments of the present invention is indicated by a solid line.
The piezoelectric speaker 40 has a effective frequency range of about 15 kHz to about 100 kHz. The piezoelectric vibrator 1 has a natural oscillation frequency of about 103 kHz, which is higher than the effective frequency range of the dome-shaped piezoelectric vibrator 1.
FIG. 3 shows the sound pressure characteristic in the frequency band of 20 kHz to 50 kHz which is particularly important in the effective frequency range.
In FIG. 3, the average sound pressure level in the frequency band of 20 kHz to 50 kHz is not necessarily sufficient. In some cases, it is desired that the sound pressure level for an entire effective frequency range or for a certain frequency band be further increased.